A typical pedicle-screw assembly is used in implants for correcting and stabilizing the spinal column. Such an assembly has a screw that is anchored in vertebral bone and has a screw head at an axial end of a threaded shaft. A tulip head or body has a screw-head seat open toward the threaded shaft for the screw head and a rod seat formed between the flanks or wings of the tulip body for fixing a spine-stabilizing rod.
A pedicle-screw assembly of this type is known from US 2007/0288004. It has a threaded shaft that can be deflected polyaxially by a maximum of 45° relative to the longitudinal tulip body axis, and an seat for the screw head that is formed by flexible fingers that grip over the equator of the ball-shaped screw head. As a result, the ball-shaped screw head is retained by being concentrically clamped with respect to the longitudinal head axis.
In US 2010/065648 a similar pedicle-screw assembly is shown that also forms a snap lock for retaining the ball-shaped screw head. The fingers reach over the equatorial line of the ball-shaped head, as a result of which polyaxial deflection by a maximum of 45° of the threaded shaft is possible relative to the longitudinal tulip body axis.
US 2005/0203516 shows a pedicle-screw assembly that can hold a rectangular-section rod. In one embodiment, the threaded shaft can be separated from the head, and the head is mounted in the threaded shaft radial to the longitudinal head axis. A slot allows deflection of the threaded shaft to the head around the longitudinal head axis. This relative rotation is intended to protect the screw from being screwed out of the bone of the spinal cord when the rod is tightened. Even here, no deflection of the threaded shaft of more than 45° is possible.
To be able to perform spinal fixation efficiently, it is necessary to deflect the threaded shaft polyaxially relative to the longitudinal head axis in order to be able to attach the rod that will later anchor the vertebrae securely at the pedicle-screw assemblys. The threaded shaft of the pedicle screw is usually screwed into the massive part of the bone, as a result of which expensive steps for bone regeneration can be avoided. The threaded shafts of the pedicle screws that are used are therefore often screwed into bones at different positions or at different angles. As a result, a displaceable the tulip body is very helpful for fixing the rod that is connecting the bones firmly and securely at the head of the pedicle screw.